1. Field of the Invention
This invention relates to centralizers for well screens. This device provides a centralizer with a longitudinal flowpath therethrough that reduces blank, non-producing tubing sections by adding more screen area. The methods relate to constructing the centralizer and for joining well screens.
2. Description of Related Art
In the past, various approaches to centering well screen tubing in a well bore have been used. The "spring steel" type of centralizers were among the first devices used. An example of this centralizer is shown on pages 7662-66 in Volume 4 of the 1985-86 Edition of the Composite Catalog of Oil Field Equipment and Services by WORLD OIL. These centralizers were clamped to the blank or non-well screen sections by one end of the centralizer and expanded and contracted up and down a short length of the outside diameter of the tubing as the tubing traveled in the well bore. Due to the light, springy nature of these devices, when they were used in deviated well bores, they had a tendency to collapse under the tubing weight allowing the tubing to drag along the side of the well bore and to become decentralized. When withdrawn from the well bore along with the well screen, these spring steel centralizers often displayed a tendency to "ball up" inside wash-over pipe used to retrieve well screens.
Other centralizers used on well screen tubing consist of usually four or more blade-like projections welded on blank tubing sections. These type of centralizers require that the well screen section be interrupted by blank tubing inserts varying in length from six inches to one foot. Depending on the deviation of the well bore, two or more of these sections may be required to center the well screen properly, thus reducing the producing area of the well screen tubing considerably.
Some centralizers are designed to be clamped onto blank tubing sections or to well screen sections and are usually two pieces bolted together to clamp around the outside circumference of the blank tubing or well screens. This type usually has four or more fin-like projections either welded on or molded into the body of the centralizer. An example of this type is shown in U.S. Pat. No. 3,981,359 by Dewitt L. Fortenberry and assigned to UOP, Inc. The present invention has neither a pair of identical housings nor the fastener members to keep the two sections together as shown. U.S. Pat. No. 4,284,138 by Richard E. Allred and assigned to UOP, Inc. shows a coated screen jacket that could include a finned centralizer welded to the tubing base. Allred's centralizer is welded to a blank tubing portion of the well screen. This device is used to reduce the expense of using stainless steel by using a coated low cost steel.
None of the above address the decrease in flow experienced by placing centralizers in a string of well screens. The present invention, unlike the above patents, increases the available flow area of the well screen tubing while allowing the use of centralizers. The blades of the present invention also add to the structural integrity of the centralizer.
A method and apparatus for joining well screens is described in U.S. Pat. No. 4,509,600 by Harry J. Boudreaux et al. and assigned to UOP, Inc. which, according to the Summary of the Invention, requires that a boss ring be attached to the well screens in order to support the well screen while the well screens are being joined to another length of well screen tubing. The present invention does not require the step of attaching a boss ring nor using the boss ring to support the well screen tubing. The boss ring is further described but not claimed in U.S. Pat. No. 4,506,730 by Chris D. McCollin, et al. which was assigned to UOP, Inc.